This invention pertains generally to a method of purifying semiconductor materials and particularly to the use of an applied electric field for the purification of semiconductor material used in radiation detectors.
Many of the commonly used radiation detectors such as Si(Li) or Ge operate most effectively at cryogenic temperatures. However, the need to keep such detectors cooled to cryogenic tempertures poses significant limitations on the use of these materials in many applications, particularly where portability is desired. The general requirement for room temperature operation of a semiconducting material as a nuclear detector and spectrometer is a relatively large band gap energy such that thermal generation of charge carriers is kept to a minimum. Conversely, the requirement for a high resolution gamma ray spectrometer is a small band gap energy such that a large number of electron-hole pairs is created for an absorbed quantum of ionizing radiation. Therefore, a compromise is necessary if a semiconducting material is to be considered for a radiation spectrometer operating at room temperature. The material under consideration should also have a relatively high average atomic number if used in gamma ray spectroscopy to increase the gamma ray interaction probability. High charge carrier mobilities and long charge carrier lifetimes are also needed to ensure efficient charge carrier extraction and minimal effects from position dependent charge collection.
In contrast to the III-V and II-VI semiconductors, such as GaAs and CdTe, which have less than ideal band gap widths and high dark currents making them unattractive candidates for room temperature radiation detection devices, certain nonmetallic, crystalline solids such as mercuric iodide (HgI.sub.2), lead iodide (PbI.sub.2), thallium bromide (TlBr), indium iodide (InI), thallium bromoiodide (TlBrI), and mercuric bromoiodide (HgBrI) are particularly useful as materials for room temperature radiation detection devices. The distinguishing properties of these materials which make them particularly attractive for use in room temperature radiation detection devices are constituents with relatively high atomic numbers, low electron-hole pair production energy and high intrinsic resistivity. As shown in Table 1, PbI.sub.2 and HgI.sub.2 (also known as red mercuric iodide or .alpha.-HgI.sub.2, the stable form of HgI.sub.2 at room temperature) are semiconductors that possess properties that make them particularly attractive as detector materials for room temperature high resolution x-ray and .gamma.-ray spectroscopic devices. Mercuric iodide, HgI.sub.2, is particularly preferred for this application.
TABLE I ______________________________________ IV, III-V and II-VI Materials HgI.sub.2 & PbI.sub.2 ______________________________________ Average Atomic No. (Z) &lt;50 &gt;60 Band Gap (eV) &lt;1.5 &gt;2.0 Pair Production Energy Band Gap 3 .apprxeq.2 Average Resistivity (.OMEGA.*cm) &lt;1 .times. 10.sup.9 &gt;1 .times. 10.sup.12 ______________________________________
One of the primary problems associated with nonmetallic, crystalline semiconducting materials lies in the presence of charge trapping defect sites and electrical instabilities caused by impurities in the starting material or introduced during subsequent processing.
Impurities, either present in the initial bulk material or introduced during detector crystal growth are believed to be one of the largest source of charge trapping defects in HgI.sub.2. These impurities affect the crystal structure and disturb the local electric field. In addition, electrically active impurities may move under the influence of an applied field leading to unpredictable and variable electrical properties including high dark current and spectral distortions. These effects lead to both immediate and long term degradation of detector performance. In order to minimize this performance degradation it is desirable to reduce impurity levels to less than 10.sup.15 /cm.sup.3. There have been numerous methods proposed for the purification of HgI.sub.2. For example, in U.S. Pat. No. 4,581,218 a method for preparing high purity HgI.sub.2 is disclosed in which HgI.sub.2 is first synthesized and then purified by at least one vacuum distillation. U.S. Pat. No. 4,282,057 discloses a method for producing high purity HgI.sub.2 employing polymer controlled crystal growth. Another method of purification of HgI.sub.2 is disclosed in U.S. Pat. No. 4,554,150 wherein chemically pure HgI.sub.2 is vaporized in an inert gas containing oxygen and iodine and subsequently reevaporated and recrystallized for further purification. Other techniques well known in the art have also been used to purify HgI.sub.2, such as zone refining and multiple recrystallizations from solution. While these methods have proven to be effective in producing HgI.sub.2 pure enough for radiation detector devices they are difficult to control, time consuming, expensive, the yield of purified material is low and they require extensive handling of hazardous materials including HgI.sub.2 itself. Many of the prior art purification methods require the use of high purity starting materials as well as the use of solvents which may be hazardous and/or difficult to dispose of or require extensive reprocessing before they may be reused. Furthermore, as the number of processing steps increases the probability of reintroducing impurities into the HgI.sub.2 also increases.
A method of purifying III-V compound semiconductors, semiconducting pseudo-binary alloys and intermetallic semiconductors such as HgCdTe by means of solid state electromigration has been disclosed in U.S. Pat. No. 4,675,087. Here, the impure semiconductor material is partially immersed in a reservoir of liquid metal compatible with the semiconductor and an electric current is passed through the sample causing impurities contained therein to migrate into the liquid metal electrode. While useful for removing impurities from metallic semiconductors, this method of purifying semiconducting materials is ineffective for nonmetallic, ionic, semiconducting compounds such as HgI.sub.2. Furthermore, the '087 invention requires the use of additional hazardous materials such as mercury or cadmium.